On the other hand, if you're lifting weights cortisol works in your favor by delivering fat to muscle tissue. "When cortisol is 'socializing' with HGH and testosterone, such as when you're intense weight training to build muscle, it aids fat loss," says Teta.

Not so with cardio. Keep your cortisol levels elevated too long with low-intensity exercise (without HGH and testosterone present), and you start to break down lean muscle tissue and store fat.

So what's the alternative to hours striding elliptical machines and aerobics classes? Weight resistance, for one. Despite what you might think, you're not going to turn into Madonna or Arnold Schwarzenegger lifting weights 20 minutes, three times a week.

You might not have the time or inclination, however, to buy free weights or frequent your local gym. Fortunately, there's an even more efficient burn-burning exercise called high-intensity interval training (HIIT).

HIIT, also called burst training, involves short, intense 30 to 60 second bursts followed by one to two minutes of active recovery where you're moving at a low intensity so you can catch your breath and stabilize your heart rate.

You don't need any special equipment. Any park (especially ones with hills) works, as does any set of stairs. You might run up the hill, walk briskly down, and repeat. Or you might run, walk, and repeat. Full throttle going up, steady coming down, and repeat: that's burst training.

You want to shoot for four to 12 total minutes of high intensity bursts. The point is to push your body to maximum capacity. If you can do burst training for more than a minute at a time, you're not doing it hard enough.

Burst training works in your favor for fat burning because it creates oxygen debt and forces recovery: your body literally must catch up for the oxygen you expended. Studies in the Journal of Applied Physiology and other journals confirm burst training increases oxygen debt for fat burning better than other kinds of exercise.

But because burst training is short and intense, you're not chronically elevating cortisol like aerobic exercise does. Like with weight resistance, you're also raising anabolic hormones like testosterone to counteract those stress hormones. Burst training also raises lactic acid, which increases human growth hormone (HGH) and supports fat burning. You've got several hormones working in your favor.